Semiconductor dies typically include a plurality of integrated circuits, bond-pads coupled to the integrated circuits, and metal routing layers for routing electrical signals between the bond-pads and external contacts. Fabricating and packaging such semiconductor dies includes forming interconnects to electrically couple the bond-pads and/or metal routing layers to external devices (e.g., a lead frame, a printed circuit board, etc.).
In some applications, the interconnects extend through at least a significant portion of the semiconductor dies (commonly referred to as “through-silicon vias” or “TSVs”). In many cases, TSVs extend completely through the dies. One conventional process for forming TSVs can include forming deep vias on the front and/or back side of a die in alignment with corresponding bond-pads. The vias are then filled with a conductive material (e.g., copper). Solder balls and/or other external electrical contacts are subsequently attached to the TSVs.
Current copper TSVs, however, are difficult to manufacture and may limit the electrical performance of advanced microelectronic devices. High-performance microelectronic devices generally have a large number of TSVs in a small space such that the pitch between TSVs is very low. One concern of copper TSVs is that the difference between the coefficient of thermal expansion (“CTE”) for silicon and the CTE for copper can lead to cracks that allow the copper to diffuse into the silicon. This may limit performance and is particularly problematic with small pitch sizes. Additionally, wafers are not being thinned as much as they were in the past to avoid breakage, and thus the vias for the TSVs need to be deeper. However, it is difficult to plate copper into such deeper vias, and therefore it is difficult to form copper TSVs in many types of devices.
Tungsten has been proposed as an alternative to copper for TSVs. However, fabricating tungsten TSVs at the production level has not been successfully implemented because it is impracticable to fill the large TSVs with tungsten and tungsten is not as conductive as copper. Therefore, tungsten is not known as a ready substitute for copper in forming TSVs.